I just finished writing a paper on this. Here are some upsides:

Studies have shown that disclosing information about oneself is an intrinsically rewarding experience [0]. Social media offerings provide a platform for sharing information easily with a large audience, which activates reward mechanisms in the brain. People use social media because it makes them feel good, which is probably the reason for their explosive growth over the last decades.

Social media offers a way to stay connected with people, or at least feel connected, without having to put in much effort. When you open the Facebook or Instagram application, it is immediately filled with recent pictures and status updates of friends and relatives. Not only does this provide you with information, it might also motivate you to contact those people again, which will then reinforce the feeling of friendship. It has been shown that having an active social circle is predictive of lower stress, increased happiness, positive attitude, and self-assessed health [1].

Patients suffering from serious mental illnesses can self-organize into peer-to-peer support groups on social media platforms. Reported benefits include greater social connectedness, feelings of belonging, and being able to share personal stories and coping mechanisms. Through this empowerment, patients can challenge the stigma associated with their condition; and potentially even improve their situation by learning from peers, and gaining insight into important health decisions and possible remedies. If peer support proves insufficient, patients can motivate each other to seek professional help. [2]

Social media platforms are one of the most accessible forms of long-distance communication. Among the reasons for this, is that they are free of charge, an account is set up in a matter of minutes, and communication is not limited geographically. Furthermore, social media enables certain groups of disabled people to communicate with individuals they are normally unable to reach. For example, deaf people usually communicate through sign language or written text. Since the number of sign language ``speakers" is low, written text is the most accessible way to communicate with others. For them, social media offers an accessible and efficient way to stay in touch with friends and relatives that are geographically far away. Christine Forsberg showed that social media use among the deaf and hard hearing increased their feeling of empowerment, when empowerment is measured in self-efficacy, self-esteem, and self-determination [3].

Happiness, positive attitude, satisfaction, connectedness, and increased (mental) health can be assumed to provide positive utility, and thus promoting them is ethical from a utilitarian viewpoint. It is worth noting however, that there is a flipside to most of the effects covered above (see section \ref{sec:negative_effects}), and it is unclear whether the cumulative utility of all positive and negative consquences is positive or not.

[0]: Diana Tamir and Jason Mitchell. “Disclosing information about the self is intrinsically rewarding”. In: Proceedings of the National Academy of Sciences of the United States of America109 (May 2012), pp. 8038–43. doi: 10.1073/pnas.1202129109.

[1]: Suwen Lin et al. “Social network structure is predictive of health and wellness”. In: PLOS ONE 14.6 (June 2019), pp. 1–17. doi: 10.1371/journal.pone.0217264. url: https://doi.org/10.1371/journal.pone.0217264.

[2]: J. A. Naslund et al. “The future of mental health care: peer-to-peer support and social media”. In Epidemiology and Psychiatric Sciences 25.2 (2016), pp. 113–122. doi: 10.1017/S2045796015001067.

[3]: Christine Forsberg. “The Empowerment of Deaf Cochlear Implant Users Through Social Media in the UK, the Netherlands, and Croatia”. MA thesis. July 2020. url: http://arno.uvt.nl/show.cgi?fid=152367.

Also, social media has been used to organize riots in the Arab spring, Hong Kong, etc.